In the production of steel, a steel melt is typically produced in a furnace and then tapped into a ladle, where it is treated with one or more ingredients for refining or alloying purposes. The steel produced in an electric arc furnace or in a basic oxygen furnace typically has a low carbon content and a high oxygen content. The oxygen content is typically reduced to a level below about 3 ppm for continuous casting. To lower the oxygen content, aluminum or silicon metal is generally added. However, addition of aluminum metal results in the formation of alumina (aluminum oxide) which is a very refractory inclusion. In a metal melt, all the inclusions typically do not float up to the surface of the molten metal and into the slag. To remove or modify these inclusions, calcium or calcium compounds such as CaC2, CaAl and CaSi, or calcium briquettes or pellets are added to the melt to form a liquid calcium aluminate inclusion such as mayelite, 12CaO.7Al2O3. Calcium also lowers the sulfur content of steel by the formation of calcium sulfides.
These materials are added to the melt in the furnace or in the ladle, or added by pouring the steel melt over the materials placed in the ladle. The amount of calcium added to the melt is relatively small as calcium has limited solubility in liquid steel, having a solubility in the 0.032% range. However, with all these methods the calcium yield is low and subject to considerable variations and, therefore, it is difficult to control the effect of calcium treatment. Furthermore, due to the low density of calcium relative to steel, and the volatility and reactivity of calcium with the molten metal, the addition of calcium to the molten metal is a complicated art.
Another approach utilizes a continuous feed of calcium or calcium composite wire enclosed within a steel sheath into the ladle or steel melt through a conduit positioned above the surface of the steel bath so as to be perpendicular to the surface of the molten bath. When this wire is introduced in a substantially vertical direction into the steel melt through the surface of the liquid slag/steel, the outer steel sheath delays the release of the low melting temperature, low density and highly reactive core materials, thereby increasing the calcium-molten steel mixing. Therefore the effectiveness of the calcium treatment is enhanced.
However, in such methods, the high volatility of calcium hinders the efficient utilization of the calcium additive. If the wire does not penetrate to a sufficient depth in the molten metal before the calcium in the wire desolidifies, a low residence time and poor utilization of the calcium results, along with a non-uniform treatment of the melt. In the case of surface-additive feeding, the additive needs to penetrate through the ladle slag. It is important that all or most of the calcium remain unreacted until the calcium descends at least to a critical depth at which the ferrostatic pressure is equal to the vapor pressure of calcium. If calcium desolidifies at ferrostatic pressures lower than its vapor pressure, large calcium vapor bubbles rise rapidly to the surface of the melt. The result is an inefficient, non-uniform treatment of the molten metal and the generation of a large amount of turbulence at the surface of the melt.
Another current approach feeds a calcium or calcium composite wire through a refractory lance submerged below the liquid steel surface. The submerged refractory lance serves to reduce the intensity of the calcium-steel reaction by introducing the solid calcium to the liquid steel at a point below the critical depth for volatilization of the calcium. This approach offers superior recovery to surface feeding of wire.
Improved metal treatment methods and apparatus are desired.